It looks like a crown conch. I used to have one, and he murdered a bunch of my snails. He would stay buried in the sand until he got hungry, then get up and kill a snail for dinner. I gave him back to the LFS.
If you can catch him when he's on the prowl, feed him something meaty like krill. I tried that for a while, but realized he was killing my snails anyway. He never came out during the daily feedings, and I couldn't always get to him in time when he was out.